~기 and ~ㅁ/음: two more ways to make a noun

Take the stem and add 기. 기 starts with a consonant, so irregular verbs keep their stems and nothing else changes. On its own, the result names an activity, a bit like English "-ing".

Example 3 is the kind of thing you'd see on a to-do list or a page heading. In full sentences, 기 mostly turns up inside fixed patterns, and the next four sections are those patterns.

Put 기 in front of a word that judges an activity: 쉽다, 어렵다, 좋다, 힘들다, 싫다. English says "easy to read", and Korean says "reading is easy". You can add 가, as in 읽기가 쉬워요, and often people don't.

기 쉽다 has a second meaning: likely to. Something that happens easily is something that tends to happen, so the step from one to the other is short. It usually describes things you'd rather avoid.

전에 from lesson 16 needs a noun in front of it, as in 수업 전에. A verb gets there with 기: 자기 전에, before sleeping. The clause with 기 전에 comes first, and the thing you did before it follows.

Example 4 has two subjects, and that's fine. The other half, "after doing", is ~ㄴ/은 후에 in lesson 40.

시작하다 takes an activity as its object, so 기 fits: 먹기 시작했어요, (I) started eating. You'll mostly see it in the past, because a start is usually reported once it's happened.

Example 4 goes through 아/어지다 from lesson 20. 덥다 is a state and 더워지다 is a change, and a change is something that can start. That's the usual route for adjectives.

도 means "also", but it attaches to nouns, so it has nowhere to go on a verb. 기 makes the noun, 도 sits on it, and 하다 comes in to carry the tense and the politeness. That's 먹기도 해요, "(I) also do eating".

The first thing this says is that an action is one option among others, so it often means "sometimes". Compare 밥도 먹어요, (I) eat rice too, which adds a thing, with 밥을 먹기도 해요, (I) sometimes have rice, which adds an action.

Used twice, with ~고 between, it gives two sides of one thing: 재미있기도 하고 힘들기도 해요. It also works on a noun with 이기도 해요, "is also".

ㅁ/음 is the other noun maker. Add ㅁ after a vowel and 음 after a consonant. ㄹ stems keep their ㄹ and put ㅁ under it, which gives you the rare double batchim ㄻ.

A lot of everyday nouns were made this way, and you know some already. They're ordinary dictionary words by now, so learn each one as vocabulary. Not every verb has one, and you can't assume the form exists just because the rule would allow it.

ㅁ/음 has a second job, and it isn't a noun you'd find in the dictionary. Put it on any stem and you have a terse statement for notes, notices, memos and online reviews. It can take 았/었 for the past.

It reads like a note to nobody, so it has no politeness level at all. That makes it fine on a whiteboard and too curt for a message to your boss. Stick to reading it for now.

Some verbs have both, and those pairs give you a rough feel for the difference. 기 tends to be the activity, and ㅁ/음 the thing you end up with. Treat that as a guide, because which form a verb has is mostly vocabulary.